xgsch2pcb configure fails in non-X environment

[Maksim Salau]

When building xgsch2pcb in non-X environment configure script fails on gtk python module check.
Example :
$ DISPLAY="" ./configure
...snip...
checking for python module gtk... no
configure: error: python module gtk is required

$ DISPLAY="" python -c "import gtk"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py", line 64, in <module>
    _init()
  File "/usr/lib64/python2.7/site-packages/gtk-2.0/gtk/__init__.py", line 52, in _init
    _gtk.init_check()
RuntimeError: could not open display

Module is found, but it tries to open display and fails.

To distinguish runtime errors from import errors, I've tried such construction:
$ python -c "import gtk" 2>&1 | grep -q 'ImportError'

But logic exit code is inverted.

Also same reason prevents configure script from getting gtk+ version. So I introduced new --disable-x option to turn that check off.

Patch is attached.